1. Technical Field
The present invention relates to a power generation system using fuel cells which directly convert chemical energy of fuel into electric energy, and particularly to a power generation system using molten carbonate fuel cells.
2. Background Art
Many power generation systems using fuel cells have been developed, and one of such conventional power generation systems is shown in FIG. 6 of the accompanying drawings. This power generation system uses natural gas as a raw material gas to be reformed to fuel gas. A fuel cell stack I generally includes a plurality of fuel cell elements stacked one after another via separators (not shown). In FIG. 6, there is shown only one fuel cell element for illustrative purpose. The fuel cell element includes an electrolyte plate 601, a cathode 602 and an anode 603, with the electrolyte plate 601 sandwiched by the cathode 602 and the anode 603. Air A is compressed by a compressor 604, cooled by a cooling device 605, further compressed by another compressor 606 and preheated by an air preheater 607 before it is introduced to the cathode 602 of the fuel cell stack I by an air feed line 608. Part of the air A is fed to a combustion chamber of a reformer 610 by a branch line 609. Gases discharged from the cathode 602 (called "cathode gas") are forced into a turbine 612 through a cathode gas line 611, then into the air preheater 607 and expelled to the atmosphere. On the other hand, fuel gas which is obtained by reforming natural gas (CH.sub.4) NG is introduced to the anode 603 of the fuel cell stack I. Natural gas NG flows through natural gas preheaters 613, 614 and a desulfurizer 615 before it reaches the reformer 610. Natural gas is reformed to the fuel gas by the reformer 610 and fed to the anode 603 by a fuel gas feed line 616. Gases discharged from the anode 603 (called "anode gas") are forced into a heat exchanger 617, the natural gas preheater 614, a steam generator 618, another natural gas preheater 613, a condenser 619 and a gas-liquid separator 620. In the gas-liquid separator 620, H.sub.2 O is separated from the anode gas, and the H.sub.2 O-removed anode gas is pressurized by a blower 621 and then introduced to a combustion chamber of the reformer 610 by a line 622 extending through the heat exchanger 617. Gases discharged from the reformer 610 are introduced to the cathode 602. H.sub.2 O (left bottom in the illustration) separated by the gas-liquid separator 620 is pressurized by a pump 623 (right in the illustration) and fed to a water heater 624. H.sub.2 O is heated to steam in the heater 624 and transferred by a line 625 via the steam generator 618 to merge with natural gas NG before it enters the reformer 610. Numeral 626 designates a blower for cathode recirculation.
When the fuel cell stack I is operated for power generation, a following reaction takes place in the reformer 610: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
On the other hand, a following reaction occurs at the cathode 602 of the fuel cell stack I: EQU CO.sub.2 +1/20.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.-
Upon this reaction, as seen in the above equation, carbonate ion CO.sub.3.sup.- is produced. The carbonate ion CO.sub.3.sup.- migrates in the electrolyte plate 601 and reaches the anode 603. Since the fuel gas FG prepared by the reformer 610 is fed to the anode 603 and the fuel gas FG contacts the carbonate ion CO.sub.3.sup.-, following reactions occur: EQU CO.sub.3.sup.- +H.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+2e.sup.- CO.sub.3.sup.- +CO.fwdarw.2CO.sub.2 +2e.sup.-
Therefore, 5CO.sub.2 and 3H.sub.2 O are discharged from the anode 603 as the anode gas.
However, the conventional power generation system has following problems:
Gas flow rates through the cell elements are not always homogeneous in the height direction (vertical direction) of the fuel cell stack as shown in FIG. 7, and gas flow rates in each cell element are not homogeneous in the width direction (horizontal direction) of the fuel cell stack if the width of the cell element (or fuel cell stack) is large as shown in FIG. 8. Because of the inhomogeneous flow rates, some fuel cell elements suffer from insufficient fuel if a high fuel utilization factor is required to the fuel cell system. This results in an under voltage (an output voltage of the system is lower than a designed or desired value). If a stable operation of the fuel cell stack is desired, i.e., if the under voltage should be avoided, the fuel utilization factor should be lowered. "LIMIT" in FIGS. 7 and 8 indicate this. In addition, since the conventional power generation system cools the fuel cell stack I with sensible heat of the cathode, the temperature of the fuel cell stack entrance cannot be set high. Consequently, the S/C ratio (steam/carbon ratio) cannot be set low (If the S/C ratio is low, deposition or precipitation of carbon will not be prevented), and generally the fuel cell stack is operated with its entrance temperature being about 570.degree. C. and the S/C ratio being about 3.
Another example of conventional power generation system using a fuel cell stack is shown in FIG. 9. The fuel cell stack I includes a plurality of fuel cell elements stacked one after another via separators (not shown) and each fuel cell element includes an anode 903, a cathode 902 and an electrolyte plate 901. The electrolyte plate 901 is a porous substance soaked with carbonate and therefore this fuel cell is called a molten carbonate fuel cell. In FIG. 9, there is illustrated one fuel cell element for illustrative purpose. Air A (oxidizing gas) is supplied to the cathode 902 of the fuel cell stack I and fuel gas FG is supplied to the anode 903 of the same. A line 905 is connected to an entrance of the anode 903 of the fuel cell stack I such that the fuel gas FG produced by a reformer 904 reaches the anode 903. Natural gas NG (raw material gas to be reformed) is desulfurized by a desulfurizer 907 on a natural gas feed line 906, preheated by a natural gas preheater 908 and introduced to a reforming chamber 904a of the reformer 904. Natural gas NG is reformed to the fuel gas FG in the reforming chamber 904a and introduced into the anode 903. On the other hand, the air A is introduced to a filter 909, pressurized by an air blower 911 on an air feed line 910, heated by an air preheater 912 and introduced to an entrance of the cathode 902 of the fuel cell stack I.
Gases discharged from the anode 903 (called "anode gas") flow into a catalyst combustor 914 through an anode gas line 913. Residual combustible matters among the anode gas (not all the matters are used in the reaction at the anode 903) are burned in the catalyst combustor 914 using part of the cathode gas (gases discharged from the cathode 902) introduced to the combustor 914 by a line branched from a cathode gas line 915. In order to direct heat produced upon this combustion to a heating chamber 904b of the reformer 904 so as to use this heat for the reforming reaction in the reforming chamber 904a, the catalyst combustor 914 and the heating chamber 904b are connected with each other by a combustion exhaust gas line 916. Another part of the cathode gas is introduced to the air preheater 912 by the cathode gas line 915 before it is expelled to the atmosphere. A cathode gas recirculation blower 918 directs part of the cathode gas to the entrance of the cathode 902 through a recirculation line 917. In order to use sensible heat of gases discharged from the heating chamber 904b of the reformer 904 for generation of steam, these gases are introduced to a steam super heater 920, a steam generator 921 and another steam generator 922 by a line 919. The gases then flow through a condenser 923 and a gas-liquid separator 924. The steam generator 922 produces steam to be used in the reformation in the reformer 904. Clean water H.sub.2 O processed by a water processor 925 flows into the gas-liquid separator 924. Water separated by the gas-liquid separator 924 is pressurized by a water feed pump 926 together with the clean water H.sub.2 O and introduced to the steam generators 921 and 922. Steam produced by the steam generator 921 is recovered by a steam recovering line 927. Steam produced by the steam generator 922 is super heated by the steam super heater 920 and fed into the natural gas feed line 906 by a steam line 928. Gases separated by the gas-liquid separator 924 flow toward the air blower 911 on the air feed line 910.
As the power generation starts using the system having the above-described structure, the natural gas NG is introduced to the reforming chamber 904a of the reformer 904 via the natural gas preheater 908 and a following reaction takes place in the reforming chamber 904a: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
CO and 3H.sub.2 are supplied as the fuel gas to the anode 903 of the fuel cell stack I. On the other hand, the air A preheated by the air preheater 912 is introduced to the cathode 902 of the fuel cell stack I and a following reaction takes place at the cathode 902: EQU CO.sub.2 +1/20.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.--
The carbonate ion CO.sub.3.sup.-- reaches the anode 903 via the electrolyte plate 901. Since the fuel gas FG has been fed to the anode 903, following reactions are caused at the anode 903: EQU CO.sub.3.sup.-- +H.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+2e.sup.- EQU CO.sub.3.sup.-- +CO.fwdarw.2CO.sub.2 +2e.sup.-
Thus, electric current flows as a certain electrical load is connected between the cathode 902 and the anode 903.
However, this type of power generation system also has shortcomings. Since the power generation system has only one reformer 904 and only one fuel cell stack I, the reformer 904 should possess a high reforming efficiency or rate. Consequently, the reforming temperature should be high. The reformer 904 generally cannot stand a high temperature with respect to a structural rigidity, and the longevity of the reforming catalyst in the reforming chamber 904a is shortened as the reforming temperature is raised. If the temperature of the reformer 904 is lowered in order to ensure an adequate longevity of the reforming catalyst and the reformer, a high reforming efficiency cannot be expected and a concentration of hydrogen introduced to the anode 903 becomes low. Therefore, it is not possible to obtain a high voltage output from the power generation system.